Using this class creates all the exceptions classes used by Alzabo
(via the Exception::Class class). If the
environment variable 'ALZABO_DEBUG' is true, then it will turn on
stacktrace generation for all the exception classes.

Alzabo::Exception
This is the base class for all exceptions generated within Alzabo (all
exceptions should return true for $@->isa('Alzabo::Exception')
except those that are generated via internal Perl errors).

Alzabo::Exception::Cache::Deleted
An attempt was made to operate on a row that had been deleted in the
cache. In this case there is no point in attempting further
operations on the row, as it is no longer in the database.

Alzabo::Exception::Cache::Expired
An attempt was made to operate on a row that had been expired in the
cache. The row will refresh itself before returning this exception so
it may be desirable to attempt the operation that caused this error
again.